This invention relates generally to image digitizing apparatus that facilitates the storage in digital computers of images recorded as spatial variations in reflectivity or transmissivity of two-dimensional storage media. More specifically, the invention relates to image digitizing apparatus that can accurately perform the digitizing function on recorded images having exceptionally wide dynamic ranges.
The pictorial data recorded in two-dimensional media can be described to any degree of desired accuracy by a two-dimensional array of picture elements ("pixels") quantified in terms of the reflectivities or transmissivities of the medium at the particular coordinates of the pixels.
The pixel density required for an adequate description of the pictorial data depends on the spatial frequency content of the data and the degree of accuracy desired in reconstructing the pictorial data from the derived pixel data. The finer the detail in the original pictorial data, the greater must be the pixel density to adequately represent the pictorial data. And the greater the pixel density, the greater must be the spatial resolution of the image digitizing apparatus, the more precise must be the mechanical operations performed by the apparatus in extracting the pixel data from the image-containing medium, and the more time consuming is the performance of the digitizing function. Thus, economic considerations dictate that the spatial resolution of the image digitizing apparatus should be no greater than the image detail demands.
The fidelity with which images can be reconstructed from pixel data depends not only on pixel density but also on the accuracy of representation of the reflectivity or transmissivity of each pixel. Solid-state light detectors, which are typically used in image digitizing equipments to convert the imaged light from pixels into electrical signals, have dynamic ranges between a few hundred and a thousand, dynamic range being the ratio of the maximum output signal of the detector when illuminated with light and the noise output in the absence of light illumination. Dynamic ranges falling within this range are adequate for many categories of image-containing media.
However, certain image categories, like medical x-ray diagnostic film, have dynamic ranges an order of magnitude greater. Image digitizing apparatus that can accommodate dynamic ranges of this magnitude generally involve the use of photomultiplier tube light detectors, a costly alternative to solid-state detectors. If a way could be found for accomplishing the image digitizing function with less-expensive solid-state light detectors, the utilization of computers in the transmission, processing, and display of medical x-ray data could be vastly expanded.
The obstacle that presently stands in the way of such progress is the limited dynamic range of solid-state detectors. Some simple means for effectively expanding the dynamic range of such detectors is needed to fully realize the diagnostic utility of medical x-ray film.